Question on cartridge length and velocity

[RM]

Take two reloaded pistol rounds, identical except one is reloaded just over the minimum acceptable OAL and one is reloaded just under the maximum acceptable OAL. Would you expect: no difference, a little, or a significant difference in the velocity of the two rounds? How much does the answer to this question depend on the caliber of the rounds? Thank you.

 

[1858rem]

i dont know for sure, i have a chrono, but blew up my pistol before i got to testing OAL for velocity differences. i am sure though if you seat too far past min OAL you will get a abnormal pressure spike. i dont think there would much more difference in velocity than you would normally have in the ES of your load. smaller cases amplify small changes, or make them more apparent, so probably could make a difference in 25 acp, but not in 45 colt if you stayed within specs.

 

[Zak Smith]

If you take a load that is safe pressure at OAL "X", it may be overpressure at an OAL shorter than "X".

 

[ants]

Well, RM, I started that very experiment with 45acp and 9mm two years ago. But I gave up.

I found that there was no single formula. When everything else is held constant, the velocity difference you measure as you change OAL is more dramatic when you are using a fast powder, but less dramatic when you are using heavier bullets, but more dramatic when you are at max load, but less dramatic for larger calibers. You have to pick your caliber, bullet, powder and charge, then test different cartridge overall lengths yourself. There is no single formula that holds true all the time.

For my favorite 9mm load of 4.2 grains Titegroup, 124g plated round nose bullet, Wolf SP primer, and 1.130" overall length: Shortening the overall length by .020 inches is equal to a tenth of a grain of powder. But that is only true for that particular combination. Every other combination will behave differently.

 

[Steve C]

I think ants pretty much described the issue. It all depends, there's no simple answer.

OAL changes in a relatively large capacity case with lots of room gives you less of a change than with a small capacity case. The type of powder makes a lot of difference if its a high density low volume powder or a low density high volume powder, fast or slow burning, compression sensitive or not sensitive. Large capacity cases the position of the powder, especially small volume charges of dense fast powder up by the bullet or down by the primer makes quite a bit of difference.

 

[1SOW]

Research I've done shows pressure changes much faster than velocity. Stay within safe published load data.

I use fast powders.

I experimented with Win 231 in 9mm and it was fairly predictable--shorter = faster AND don't forget higher pressures..

I did the same with Vit and it wasn't nearly so linear. I lengthened COLs where in 231 I would have seen a steady change, but in VIT it showed very little/no change with my light loads.